Rapid Particle Solidification
This invention relates generally to the production of powders from a liquid melt by atomization and solidification. More particularly it relates to the preparation of higher temperature materials in finely divided form by fluid atomization and to the apparatus in which such process is performed and the product obtained by the process.
For example it may be applied to the production of powders from melts of superalloys.
There is a well established need for an economic means of producing powders of superalloys. Such powders can be used in making superalloy articles by powder metallurgy techniques. The present industrial need for such powders is expanding and will continue to expand as the demand for superalloy articles expands.
Presently only about 3% of powder produced industrially is smaller than 10 microns and the cost of such powder is accordingly very high.
A major cost component of fine powders, prepared by atomization and useful in industrial applications, is the cost of the gas used in the atomization. At present the cost of the gas increases as the percentage of fine powder sought in an atomized sample is increased. Also as finer and finer powders are sought the quantity of gas per unit of mass of powder produced increases. The gases consumed in producing powder, particularly the inert gases such as argon, are expensive.
There is at present a growing industrial demand for finer powders. Accordingly there is a need to develop gas atomization techniques and apparatus which can increase the efficiency of converting molten alloy into powder, and to conserve the gas consumed in producing powder in a desired size range, particularly where the desired size range are growing smaller and smaller.
The production of fine powder is influenced by the surface tension of the melt from which the fine powder is produced. For melts of high surface tension production of fine powder is more difficult and consumes more gas and energy. The present typical industrial yield of fine powder of less than 37 micrometers average diameter from molten metals having high surface tensions is of the order of 25 weight % to about 40 weight %.
Fine powders of less than 37 micrometers (or microns) of certain metals are used in low pressure plasma spray applications. In preparing such powders by presently available industrial processes as much as 60-75% of the powder must be scrapped because it is oversize. This need to selectively remove only the finer powder and to scrap the oversize powder increases the cost of usable powder.
Fine powder also has uses in the quickly changing and growing field of rapid solidification materials. Generally the larger percentage of finer powder which can be produced by a process or apparatus, the more useful the process or apparatus is in rapid solidification technology.
It is known that the rate of solidification of a molten particle of relatively small size in a convective environment such as a flowing fluid or body of fluid material is roughly proportional to the inverse of the diameter of the particle squared.
The following expression is accordingly pertinent to this relationship: ##EQU1## where T.sub.p is the rate of cooling of the particle and
D.sub.p is the particle diameter.
Accordingly, if the average size of the diameter of the particles of the composition is reduced in half then the rate of cooling is increased by a factor of about four. If the average diameter is reduced in half again the overall cooling rate is increased sixteen fold.
It is desirable to produce powders of small particle size for some applications particularly those in which the rate of cooling of the particle is significant to the properties achieved. For example there is a need for rapidly solidified powders of size smaller than 37 microns and particularly for the production of such powders by economic means.
In addition, for certain applications it it important also to have particles which have a small spectrum of particle sizes. Accordingly, if particles of a 100 micron size are desired for certain applications a process which produces most of the particles in the 80-120 micron range would have a significant advantage for many applications of such particles as compared for example to a process which produces most particles in the 60 to 140 micron range. There is also a significant economic advantage in being able to produce powder having a known or predictable average particle size as well as particle size range. The present invention improves the capability for producing such powder on an industrial scale.
If particles of 100 micron size are produced by a first process from a given molten liquid metal for a given application, and it is then learned how to produce particles with a 50 micron average size, this second process would permit a much more rapid cooling and solidification of the particles formed from the same molten liquid metal. The present invention teaches a method by which smaller particles may be formed in higher percentage from melts, including molten liquid metal. A more rapid solidification rate of such particles is achieved by this novel process partly because the particles produced are themselves smaller on the average and also because the production is repeatable and reproducible on an industrial scale.
The achievement of small particle size is advantageous for rapid cooling and for the attendant benefits which derive from rapid cooling of certain molten materials. Novel amorphous and related properties may be achieved in this way. The present invention makes possible the production of powders with such small particle size with attendant rapid cooling.
The powder metallurgy technology presently has a need for fine and ultrafine particles and particles in the size range of 10 to 37 microns in diameter. Particles having average particles in the particle size range of 10 micron to 37 micron are produced by this novel process of this invention.
The attainment of the smaller particle size may be found important in consolidation of the material by conventional powder metallurgy inasmuch as it has been observed that powder of smaller particle size can result in higher sintering rate. Also it can be significant in the consolidation of the small particle size material with a material of larger particle size where such consolidation is found desirable based on higher packing density.
Present trends in powder metallurgy are creating great interest in fine metal powders, that is, in powders having diameters less than 37 microns in diameter and also in ultrafine powders specifically powders having diameters of less than 10 microns. High surface tension in a melt material makes the formation of smaller size particles more difficult.
Conventional apparatus for producing powder from molten metals by atomization results in products depending on preparation methods and materials which have relatively broad spectra of particle sizes. The broad spectra of particle sizes are represented in FIG. 3 by the curves A, B, C and D. From examination of these curves it is evident that the particles range all the way from particle sizes of less than 10 micron to more than 100 microns. The percentage of particles of fine powder, i.e. less than 37 micron) produced by conventional technology is the range of about .about.0 to 40%, and the percentage of ultrafine powder, i.e. less than &lt;10 micron, produced is in the range of .about.0-3%. Because of the low yield of the smaller particle powder which is formed in such products the cost of the production of the ultrafine powder can be excessive ranging up to hundreds and even thousands of dollars per pound.
The graphs of FIG. 3, and illustratively curve E of FIG. 3, shows that the range of particle sizes produced by the methods of this invention when operated in a fine powder mode are significantly better than the particle size range of existing conventional processes. The data on which the curves A, B, C and D of FIG. 3 is based is from a review article by A. Lawly, "Atomization of Specialty Alloy Powders" which appeared in the January 1981 issue of Journal of Metals.
The data in the Journal of Metals article, and for the Curves A, B, C and D is for powder formed from melts of superalloys. The data from which Curve E was prepared was also data from the preparation of powder from a superalloy melt so that the two sets of data are quite comparable.
It is known that there are large differences in the ease with which powder can be prepared from different families of alloys.